1. Field of the Invention
The present invention relates to a tuning circuit with tunnel junction elements and a superconductor integrated circuit comprising the tuning circuit. Broad band tuning circuit in submillimeter wave region is needed in a radio telescope etc. The present invention provides a tuning circuit with tunnel junctions comprising frequency characteristics of small return loss in broad band.
2. Description of the Related Art
As prior art, it is known that is a tuning circuit composed of a signal source, a microstrip line of quarter-wavelength for impedance matching and a Superconductor-Insulator-Superconductor (SIS) junction element of half-wavelength or full wavelength as mixer.
FIG. 14 shows an example of prior art tuning circuit with SIS junction element. As shown in FIG. 14, the tuning circuit is composed of a signal source 1′, a quarter-wavelength strip line 2 and a full-wavelength SIS junction element 10. The full-wavelength SIS junction element 10 has a tunnel junction and superconductor electrodes, and connected to the quarter-wavelength microstrip line 2. The end terminal of the full-wavelength SIS junction element 10 is open circuit. The inner impedance of the signal source is Z0, and connected to the quarter-wavelength microstrip line and the earth.
FIGS. 15A and 15B show an example of a structure of the prior art tuning circuit. FIG. 15A shows a plain view and FIG. 15B shows a cross sectional view. The tuning circuit shown in FIGS. 15A and 15B is fabricated by following process. Trilayers of Nb/AlOx/Nb are layered on a substrate 40. The upper Nb and the AlOx layers are etched to form the upper electrode 41 and the tunnel junction 42 as shown in FIG. 15B using photo-resist mask and photolithography technique. The lower Nb layer is the lower electrode 43. An insulation layer 73 is layered over the substrate 40, the lower electrode 43 and a photo-resist mask (not shown) remained on the upper electrode 41. Further the photo-resist is lift off to form the contact hole contacting a wiring layer 70 to the upper electrode 41. The wiring layer 70 of Nb is deposited on the insulation layer 52,73 so as to contact the upper electrode 41. The lower electrode 43, the tunnel junction 42 and the upper electrode 41 compose a SIS tunnel junction element. The lower electrode 43 acts further as antenna 72. The wiring layer 70 acts as an electric line contacted to the upper electrode 41 of the SIS tunnel junction element, a upper strip line of the quarter-wavelength microstrip line 2 and an antenna 71.
In FIGS. 15A and 15B, the full-wave length SIS junction element 10 is composed of the lower electrode 41, the tunnel junction 42 and the upper electrode 43. The length of the full-wavelength SIS junction element 10 is a full-wavelength of the input signal at a center frequency, and the length of the quarter-wavelength microstrip line 2 is a quarter of a full-wavelength of the input signal. The width of full-wavelength junction element 10 is 0.6 μm, and the length is 8.68 μm. The width of the microstrip line is 3.5 μm and the length is 44.2 μm which is equal to a quarter of the input signal wavelength.
FIGS. 16A and 16B show characteristics of the tuning circuit of FIGS. 15A and 15B. FIG. 16A is Smith chart which shows an impedance locus obtained by changing input signal frequency. FIG. 16B shows characteristics of frequency to return loss for input signal power. The return loss of −10 dB in FIG. 16B corresponds to a circle of reflection coefficient of 0.3 on the Smith chart of FIG. 16A. The inner impedance Z0 of the signal source is 40Ω.
A tuning circuit of the reflection coefficient of less than −10 dB is needed usually. As shown from FIG. 16B, the frequency band of return loss of less than −10 dB of the prior art tuning circuit is about 650 GHz–675 GHz.
The frequency band of tuning circuit of the prior art shown in FIG. 14 is decided with Q factor of the SIS junction element. The frequency band is narrow, when the Q factor of the element is high. When higher the Q, the frequency band is narrower. The Q factor of SIS junction element follows from junction current density. When the junction current density is high, the Q factor is small, and contrarily when the junction current density is small, the Q factor is large. Thus, if the junction current density is controlled so as to be high, the broad band for the return loss of less than −10 dB is realized. Usually the high current density of the junction cannot be realized easily using junction fabrication technologies of the prior art. Thus it is very difficult to realize the tuning circuit of the broad band, using the prior art.